This invention relates to the testing of individual battery cells in a battery backup system having a plurality of battery cells arranged in parallel strings of serially connected battery cells and, more particularly, to a method and apparatus for determining to which battery cell in a string of battery cells a test instrument is connected.
Large battery systems are commonly used to provide backup power in case there is a failure of the commercial power grid. Typically, such a backup system includes a single string, or a plurality of parallel strings, of serially connected rechargeable battery cells and a charger connected to the commercial power grid for maintaining the charge on the battery cells. An inverter is coupled between the strings of battery cells and the load, which inverter is enabled upon the detection of a failure of the commercial power grid. In some applications, the inverter may be continuously operational to power the load with energy from the charger during the time that commercial power is available. Many of these battery backup systems, called “uninterruptible power supplies” (UPS), are configured such that the load is never aware of any failure of the commercial power grid because the battery system immediately supplies the necessary energy upon detecting a failure of the commercial power grid.
A typical installation of such an uninterruptible power supply is between the commercial power grid and a large computer system used by financial, communications, manufacturing and other commercial industries. If the battery system is taken “off-line” for any reason, the necessary protection against a power outage is lost for the time that the battery system is not connected plus the time for recharging, if a significant amount of charge has been removed during the off-line period of time. However, such battery backup systems must be monitored on a regular basis to insure that protection from commercial power grid failure is always available. Therefore, systems have been developed to perform such monitoring while the battery backup system remains on-line.
Impedance measurement is a method by which the condition of a battery cell may be assessed without taking the battery system off-line. Impedance measurements typically impose a current (hereinafter called the “loading current”) on the battery cell being evaluated and measure the resulting voltage. Various commercially available test instruments function this way. Using Kelvin connections, these instruments impose a current on just the battery cell being measured. After a measurement has been made, the operator moves the Kelvin clips to the next battery cell, reads the value, moves the clips to the next battery cell, and continues in this manner until all the battery cells have been measured. The test instrument stores all the readings it has taken but, until now, did not know to which battery cell in the string it was connected, since the operator is not constrained to any particular order in which to check the battery cells. It would therefore be desirable to have a method and apparatus embodied in a test instrument for determining to which battery cell in a string of battery cells the test instrument is connected.